⚗️ Chromium(VI) and total chromium in water and wastewater — spectrophotometry with 1,5-diphenylcarbazide (540 nm)

Physicochemistry PN-77/C-04604/08

Overview

Status of the standard: PN-C-04604-08:1977 (formerly designated PN-77/C-04604-08) has been withdrawn, but the method remains in laboratory practice.

PN-77/C-04604-08 "Woda i ścieki — Badania zawartości chromu — Oznaczanie chromu sześciowartościowego (Cr6+) i trójwartościowego (Cr3+)" [Water and waste water — Determination of chromium — Determination of hexavalent (Cr6+) and trivalent (Cr3+) chromium] is a withdrawn standard, but the 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) still names PN-C-04604-8 as the reference method for hexavalent chromium in wastewater (molecular absorption spectrophotometry / photocolorimetry), and laboratories quote it in their scopes of accreditation. THE CATALOGUE ENTRY OF THIS STANDARD GIVES NO SUCCESSOR — and that is an observation, not a conclusion. A search of the catalogue by the SUBJECT of the determination, rather than by the field "Superseded by", shows that the CURRENT document on the same subject is PN-EN ISO 18724:2026-04 "Jakość wody — Oznaczanie chromu(VI) rozpuszczonego w wodzie — Metoda fotometryczna" [Water quality — Determination of dissolved chromium(VI) in water — Photometric method] (published on 7 April 2026, technical committee KT 121), which supersedes PN-EN ISO 23913:2009 and PN-EN ISO 18412:2007. The catalogue does NOT link it to PN-C-04604-08 even once, so we do not claim that it supersedes it — we state only that a current standard exists on the same subject and with the same final determination technique. A laboratory that has PN-C-04604-08 in its scope of accreditation ought to know this before concluding that there is no alternative. The content of this entry is based on the PWr (Wrocław University of Science and Technology) laboratory manual "Spektrofotometryczne oznaczanie różnych form chromu w próbkach środowiskowych" [Spectrophotometric determination of the various forms of chromium in environmental samples], which describes this methodology.

Chromium occurs in water in the oxidation states +3 and +6 (at high salinity and pH ≈ 8 also +4); Cr(VI) compounds are readily soluble, mobile and bioavailable, and Cr₂O₇²⁻ is strongly toxic, whereas Cr³⁺ causes no damage; the permissible chromium content of drinking water is 50 ppb, and above 100 ppb chromium may harm biological activity; high concentrations occur in the waste of the metallurgical and tanning industries, and chromium in rivers passes quickly into the bottom sediments (as much as 1200 ppm). The method consists in determining Cr(III) and Cr(VI) together as total chromium after the oxidation of Cr(III) to Cr(VI), and in determining Cr(VI) alone after the removal of Cr(III); Cr(III) is found from the difference. For the determination of Cr(VI) the colorimetric method with 1,5-diphenylcarbazide is used (range 0.01–10 ppm): in an acid medium it reacts with Cr(VI) to give a red-violet compound whose intensity is proportional to the Cr(VI) content; the mechanism probably consists in the oxidation of the diphenylcarbazide to diphenylcarbazone, which forms an intensely coloured complex with the unhydrated Cr(III) ions arising in statu nascendi. The method is very sensitive and selective; interferences come from Fe, Cu, V and Mo ions, from organic substances (oxidisability above 15 mg O₂/dm³) and from turbidity above 20 mg/dm³.

THE CHOICE OF THE WRITING CONVENTION AND THE SCALE OF CITATIONS (23.09.2026). The search key had the new form ("PN-C-04604-08") and gave 2 entries at 2 laboratories. In the scopes the old scheme predominates, and within it three different sheet separators: "PN-77/C-04604/08" 22 times, "PN-77/C-04604.08" 6, "PN-77/C-04604-08" 5. After the key was switched to "PN-77/C-04604/08" the tab shows 22 entries at 17 laboratories.

A WIDER KEY WAS DELIBERATELY NOT CHOSEN. The prefix "PN-77/C-04604" would cover all three separators and give 40 entries at 29 laboratories, but it would also pull in SHEET 02 of the same series — and that is a different determination, a different method and a different parameter. So a smaller but clean reach was chosen: eleven records of sheet 08 with the dot and the hyphen separator remain outside the matching, which is the price of this decision and is written out here so that nobody takes 22 for the whole scale.

Method principle

The sample (e.g. industrial wastewater) is filtered through a soft filter, the first 20 cm³ of the filtrate being discarded, and 20.00 cm³ portions are taken into two beakers and diluted with water to about 50 cm³ — total chromium is determined in one of them and Cr(VI) in the other. Total chromium: add 0.5 cm³ of concentrated H₂SO₄, heat to boiling, add 4 % KMnO₄ dropwise to the hot solution until a distinct violet colour appears, and keep boiling for 5 min; destroy the excess KMnO₄ with very small portions of 0.5 % sodium azide until the colour disappears, boil for 5 min, cool, transfer quantitatively to a 100 cm³ flask, add 1.0 cm³ of concentrated H₂SO₄ and 0.30 cm³ of concentrated H₃PO₄, make up to the mark and mix; after 5 min add 2.0 cm³ of 0.5 % diphenylcarbazide in acetone, mix and measure the absorbance at 540 nm within 15 min against a standard containing no chromium (if the sample is yellow after cooling — dilute it tenfold and repeat with 10 cm³). Cr(VI): add 0.5 cm³ of 50 % zinc sulfate to the sample, 2 drops of phenolphthalein and, in small portions, 10 % NaOH until a pale pink colour appears (precipitation of Cr(III)), leave to clarify, filter, wash the precipitate with water, neutralise the filtrate with H₂SO₄ (1:1) against indicator paper, transfer to a 100 cm³ flask, add 1.0 cm³ of concentrated H₂SO₄ and 0.3 cm³ of concentrated H₃PO₄ (masking of iron), make up, after 5 min add 2.0 cm³ of diphenylcarbazide and measure at 540 nm within 15 min. Standards: into 50 cm³ flasks 0; 1.0; 2.0; 3.0; 5.0; 10.0; 15.0; 20.0 cm³ of a solution of 0.005 mg Cr/cm³ (0–0.100 mg Cr), 1.0 cm³ of H₂SO₄ and 0.3 cm³ of H₃PO₄, made up to the mark, 1.0 cm³ of diphenylcarbazide; the standards are stable for 15 min; the curve is measured at 540 nm. The Cr(III) content = total chromium − Cr(VI), the result being given in mg/dm³.

Applications

• Industrial wastewater (electroplating shops, tanneries, the metallurgical industry) — Cr(VI) as a limited parameter (reference method under Dz.U. 2019 poz. 1311)
• Surface water and groundwater near landfills and industrial plants — speciation of Cr(III)/Cr(VI)
• Drinking water — control of chromium (limit 50 ppb) by a colorimetric method in laboratories without AAS/ICP
• Control of Cr(VI) reduction processes in treatment plants for electroplating wastewater
• Speciation analysis: total chromium (after oxidation with KMnO₄) and Cr(VI) (after precipitation of Cr(III) as the hydroxide)

Key parameters

Standard

Standard number

PN-C-04604-08:1977

Title (PL)

Woda i ścieki — Badania zawartości chromu — Oznaczanie chromu sześciowartościowego (Cr6+) i trójwartościowego (Cr3+)

Title (EN)

Water and waste water — Determination of chromium — Determination of hexavalent (Cr6+) and trivalent (Cr3+) chromium

Step-by-step procedure

1. Preparation of the sample

   Filter the sample through a soft filter into a clean, dry beaker, discarding the first 20 cm³; pipette 20.00 cm³ of the filtrate into each of two 200 cm³ beakers and dilute with water to about 50 cm³.

2. Total chromium — oxidation

   Add 0.5 cm³ of concentrated H₂SO₄, heat to boiling, add 4 % KMnO₄ dropwise to the hot solution until a distinctly violet colour appears and boil for 5 min; destroy the excess KMnO₄ carefully with very small portions of 0.5 % NaN₃ until the colour disappears and boil for 5 min.

3. Total chromium — colour development and measurement

   After cooling, transfer quantitatively to a 100 cm³ flask, add 1.0 cm³ of concentrated H₂SO₄ and 0.30 cm³ of concentrated H₃PO₄, make up with water and mix; after 5 min add 2.0 cm³ of 0.5 % diphenylcarbazide, mix and measure the absorbance at 540 nm within 15 min against a standard containing no chromium. If the sample is yellow after cooling — dilute it tenfold (10 cm³ → 100 cm³ with the acids) and repeat.

4. Cr(VI) — removal of Cr(III)

   To the second portion add 0.5 cm³ of 50 % zinc sulfate, 2 drops of phenolphthalein and, in very small portions, 10 % NaOH until a pale pink colour appears; mix, leave to clarify, filter through a soft filter and wash the precipitate with water; neutralise the filtrate with H₂SO₄ (1:1) against indicator paper.

5. Cr(VI) — colour development and measurement

   Transfer quantitatively to a 100 cm³ flask, add 1.0 cm³ of concentrated H₂SO₄ and 0.3 cm³ of concentrated H₃PO₄, make up with water; after 5 min add 2.0 cm³ of diphenylcarbazide, mix and measure the absorbance at 540 nm within 15 min.

6. Calibration curve and result

   Into 50 cm³ flasks add 0; 1.0; 2.0; 3.0; 5.0; 10.0; 15.0; 20.0 cm³ of the solution of 0.005 mg Cr/cm³ (0–0.100 mg Cr), 1.0 cm³ of H₂SO₄ and 0.3 cm³ of H₃PO₄, make up, add 1.0 cm³ of diphenylcarbazide and measure at 540 nm within 15 min. Read the total chromium and the Cr(VI) from the curve; Cr(III) = the difference; results in mg/dm³.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Cr(VI) compounds are carcinogenic and strongly toxic — gloves, goggles, work in a fume hood, chromium waste to be disposed of
• Sodium azide — a strong poison; add it in very small portions to the hot solution until the violet colour disappears, and add no excess; do not combine it with metals or acids outside the procedure
• Concentrated sulfuric and phosphoric acids and potassium permanganate — corrosive and oxidising; take care when boiling
• Acetone (the solvent of the reagent) — flammable

Frequently asked questions